"Based on today's technology, it would take at least a half-million years to cover the likely distances to other habitable worlds"

The following equation shows how the statement above was calculated.

Distance
This distance encompasses approximately 30 "HabCat" star systems, where "HabCat" means that the star system satisfies conditions where there might be a habitable planet zone. Since it is uncertain whether any of these systems actually have habitable planets, using a threshold of 30 systems seems a reasonable first guess that one habitable planet might actually be there.
>>> [see more about the search for habitable planets]

Speed
This is the speed of the Voyager-1 spacecraft as it left our solar system. Although built of 'old' 1977 technology, it still serves as an approximate measure of our ability today. While propulsion technology has advanced somewhat since then, Voyager is only a tiny, low-powered probe. For humanity to reach another star system, a spacecraft would have to be large enough to support a large number of people.
>>> [see more about interstellar spacecraft]

Units
The distance is shown in units of "light-years" which is the distance that light covers in one year. The spacecraft speed is shown in the units of "AU per year" which means how many "Astronomical Units" are covered in one year, and where an "Astronomical Unit" is the average distance between the Earth and our Sun. The parenthetical term showing "AU per ly" is the conversion between the units of light-years and Astronomical Units.